The Dark Four Heavenly Kings (暗黒四天王 Ankoku Shiten'nō) are a group of four alien generals that serve under Alien Empera and is a smaller branch of the Empera Army. Their main base of operation is the Empera's Darkness Fear.
Founded by Alien Empera, the team's main purpose is to serve him in his conquest to invade planets. One of them, Ark Bogar attacked multiple planets that rejected Alien Empera's leadership out of jealousy of the emperor until the latter sealed him in a black hole.
Soon in the series, his position was taken by Yapool, whom wanted revenge on the Ultra Warriors and sees this as an opportunity. After his death by Mebius, Grozam, and Deathrem tried to take actions but in the end their plots were foiled by Mebius himself with the help of GUYS and Ultra Brothers.
The last member, Mefilas manage to successfully brainwash Japan into thinking Mebius as their enemy and at the same time using a loophole in one of the Ultra Brother's policies (not to attack aliens that don't endanger humanity), causing the original Ultraman not to take any actions until his attempt to kill the hypnotized GUYS shattered his entire plan. Eventually, finding the odds even, Mefilas peacefully retreated which also resulted in his demise by Empera as a punishment.
Ultraman Mebius Gaiden: Ghost Rebirth
50 years passed and in the Monster Graveyard, the members of Dark Four Heavenly Kings are revived as ghosts. Ultraman Hikari joined them for unknown reasons and together captured Ultraman Taro and Ultraman Ace, forcing Mebius and Mechazam to go to the Valley of the Flames to retrieve the Giga Battle Nizer. Using the staff, they plan to revive Alien Empera by using his remains in Mechazam but Hikari revealed his treachery and with the three Ultra Brothers they put an end to the Four Heavenly Kings again, with Mebius forced to kill Mechazam out of guilt.
- Mebius Killer (Yapool's servant/host)
- Gromite (Mefilas III's decoy)
- Inpelaizer (G)
- Ultraman Hikari (Undercover)
- Their membership of four aliens parallels the Alien Union; another group who worked under Yapool.
- In two instances of irony; Mefilas was meant to be a member of latter group and Yapool ended up being the newest addition to the Heavenly Kings.
- Instead of Mefilas, there were also plans to include two other aliens; a female and an English gentleman-based alien. If the two drafts were carried out, the Dark Four Heavenly Kings would entirely consist of new aliens.